As a flowmeter for measuring a flow rate of a liquid moving a tube, a clamp-on ultrasonic flowmeter is known. The clamp-on ultrasonic flowmeter is a flowmeter which is placed on an outer surface of a wall of a tube, and which enables to measure the flow rate of a liquid moving in the tube from the outside.
A constitution of a known flow rate-measuring system utilizing a clamp-on ultrasonic flowmeter is illustrated in FIG. 1 in the form of a section view. The clamp-on ultrasonic flowmeter is composed of a pair of ultrasonic generating-detecting devices 1a, 1b. The ultrasonic generating-detecting device 1a is composed of a ultrasonic transducer 2a and a ultrasonic propagating element 3a. The ultrasonic propagating element 3a has a bottom surface 4a and a slanting surface 5a extending from one edge of the bottom surface 4a at an acute angle. The ultrasonic transducer 2a is placed on the slanting surface 5a of the ultrasonic propagating element 3a. As the ultrasonic transducer is employed a piezoelectric transducer (vibrator). The piezoelectric transducer is composed of a piezoelectric ceramic element and a pair of electrodes for applying an electric voltage to the piezoelectric ceramic element. Similarly, the ultrasonic generating-detecting device 1b comprises a ultrasonic transducer 2b which is placed on the slanting surface of 5b of the ultrasonic propagating element 3b. 
Each of the ultrasonic transducers 2a, 2b generates a ultrasonic wave when an electric voltage is applied thereto, while it produces an electric voltage when it receives ultrasonic wave. Accordingly, the ultrasonic generating-detecting device 1a, 1b equipped with a ultrasonic transducer can function as a ultrasonic wave generator and a ultrasonic wave detector.
The ultrasonic generating-detecting devices 1a, 1b are provided on a tube 6 in such manner that the ultrasonic waves transmitted by the devices 1a, 1b propagate across the fluid which flows inside of the tube in the direction indicated by arrow 7, that is, on the route 9 (indicated by a dotted line) in the directions indicated in FIG. 1.
The flow rate of the fluid flowing inside of the tube is determined by the following method. First, a voltage pulse is applied to the ultrasonic transducer 2a of the ultrasonic generating-detecting device 1a, so as to generate a ultrasonic wave. The ultrasonic wave propagates in the ultrasonic propagating element 3a, wall of tube 6, fluid, wall of tube 6, and ultrasonic propagating element 3b on the route indicated in FIG. 1 by the dotted line 9. Subsequently, the ultrasonic wave is received by the ultrasonic transducer 2b of the ultrasonic generating-detecting device 1b. A period of time (T1) from the time when the ultrasonic wave is generated by the ultrasonic generating-detecting device 1a to the time when the ultrasonic wave is received by the ultrasonic generating-detecting device 1b is detected.
Subsequently, a voltage pulse is applied to the ultrasonic transducer 2b of the ultrasonic generating-detecting device 1b, so as to generate a ultrasonic wave. The ultrasonic wave is then propagate on the same route, but in the opposite direction, and the ultrasonic transducer 2a of the ultrasonic generating-detecting device 1a receives the propagated ultrasonic wave. A period of time (T2) from the time when the ultrasonic wave is generated by the ultrasonic generating-detecting device 1b to the time when the ultrasonic wave is received by the ultrasonic generating-detecting device 1a is detected. The period of time (T1) required for the propagation of ultrasonic wave from the device 1a to the device 1b along the arrow 9a differs from the period of time (T2) required for the propagation of ultrasonic wave from the device 1b to the device 1a along the arrow 9b. 
The period of time (T1) is shorter than a period of time required for propagating ultrasonic wave in still water because the ultrasonic wave from the device 1a to the device 1b is propagated (in the direction of the arrow 9a) at an increased rate by the aid of the flowing fluid, while the period of time (T2) is longer than a period of time required for propagating ultrasonic wave in still water because the ultrasonic wave is propagated from the device 1b to the device 1a (in the direction of the arrow 9b) against the stream of the fluid.
Therefore, the difference of the propagation period (T2−T1) is relative to the rate of movement of the fluid flowing in the tube. Therefore, the rate of movement of the flowing fluid is calculated from the difference of propagation period and separately prepared calibration data which indicate a relationship between a flow rate and a difference of propagation period.
Thus, the clamp-on ultrasonic flowmeter is advantageous in that it can determine the flow rate with no direct contact with the flowing fluid. On the other hand, the clamp-on ultrasonic flowmeter has a disadvantage in that it gives measurement data of low accuracy when it is employed for measuring a flow rate of a liquid moving in a tube having a small inner diameter. In the case that the inner diameter of the tube is small, the distance along which the ultrasonic wave is transmitted in the fluid is short, and hence the above-mentioned difference of period of time is very small. Accordingly, the accuracy of the measurement of flow rate lowers.
As is described above, the conventional ultrasonic flowmeter measures the flow rate by utilizing a ultrasonic wave which is transmitted in the fluid. If the tube is not full of the fluid or non-uniform phases such as bubbles or floating materials are present in the fluid, the ultrasonic wave is reflected or diffused by the air phase in the tube or bubbles and the like in the fluid. Therefore, it is not possible to measure the flow rate accurately.
In the measurement utilizing a clamp-on ultrasonic flowmeter, the route along which the ultrasonic wave is transmitted is defined and the distance of the route is defined by the angle of incidence and angle of refraction on the interface between the ultrasonic wave-propagating element and the tube and the angle of incidence and angle of refraction on the interface between the tube and the fluid.
Therefore, the distance of transmitting the ultrasonic wave in the fluid can be made longer by setting the angle of incidence of ultrasonic wave at a larger value (setting the angle of slanting surface of the ultrasonic wave-propagating element against the bottom surface at a larger value). However, if the angle of incidence of ultrasonic wave is set at a large value, the ultrasonic wave is liable to be reflected on the interface and the ultrasonic wave hardly enters the fluid. Further, if the angle of incidence exceeds a certain value, the ultrasonic is totally reflected and does not enter the fluid.
Otherwise, the angle of refraction of ultrasonic wave can be set at a large value and the distance of transmitting the ultrasonic wave in the fluid can be prolonged, by choosing appropriate materials of the ultrasonic-propagating element and the tube. However, there are only limited materials which are employable for manufacturing the ultrasonic propagating element. Accordingly, it is difficult to prolong the distance of transmitting ultrasonic wave in the fluid more than a certain limit.
For the above-described reasons, it is said that a commercially available clamp-on ultrasonic flowmeter can be utilized for a tube having an inner diameter of 25 mm or more.
Tubes having a small inner diameter is generally used for moving a small amount of fluid in the industrial fields of preparations of foodstuffs, pharmaceuticals, chemical products, and semiconductor devices. In these industrial fields, it is desired to accurately measure a flow rate of a fluid moving in the tube having a small inner diameter. A tube having a small inner diameter is also employed for continuously administering a pharmacologically active liquid or blood into a patient under treatment. In that case, the flow rate of a pharmacologically active liquid or blood should be measured very accurately.
Heretofore, the flow rate of a fluid moving in a tube having a small inner diameter is measured, for instance, by means of a variable area flowmeter. In the measurement employing the variable area flowmeter, a float is placed in a route which is vertically extending from the flow tube, and a height of the float moved by the ascending flow of the fluid is measured for determining the flow rate of the fluid. Thus, it is necessary that the variable area flowmeter be placed vertically so that the float can be moved up-and-down. Moreover, if a variable area flowmeter is employed for an already assembled tube lines, it is necessary to replace a part of the tube lines with the area flowmeter.
It is also known that an electromagnetic flowmeter can be employed for measuring a flow rate of a fluid moving in a tube having a small inner diameter. However, the electromagnetic flowmeter cannot be employed for measuring a fluid having no electroconductivity. Moreover, if an electromagnetic flowmeter is employed for an already assembled tube lines, it is necessary to replace a part of the tube lines with the electromagnetic flowmeter.
In addition, it is known that a ultrasonic flowmeter utilizing a ultrasonic transducer in the form of ring is employable for measuring a fluid moving in a tube having a small tube. The ultrasonic flowmeter utilizing a ultrasonic transducer in the form of ring is described in a paper entitled “Sensor arrangement and flowing characteristics of a ultrasonic micro flowmeter for liquid” by ISHIKAWA, Hiroo, et al., Collective Papers of Society of Measurement Automatic Control, 2000, Vol. 36, No. 12, pp 1071-1078.
The above-mentioned ultrasonic flowmeter comprises a pair of ultrasonic transducers in the form of ring. Each of the pair of ultrasonic transducers is arranged around a tube in which a fluid is to be moved. The measurement of a flow rate of a fluid flowing in the tube by means of the ultrasonic transducers in the form of ring is carried out by the following procedures. Initially, a ultrasonic wave is generated in one ultrasonic transducer. The ultrasonic wave is then transmitted in the fluid along the tube to reach another ultrasonic transducer. The time of period of transmission is then measured. Subsequently, a ultrasonic wave is generated in the latter ultrasonic transducer. The ultrasonic wave is then transmitted in the former ultrasonic transducer. The period of time of transmission is again measured. The difference between these periods of time of transmission is calculated. Comparison between the difference of the periods of time of transmission and a previously prepared calibration data indicating a relationship between a flow rate and a period of time of transmission teaches a flow rate of the target fluid.
In the measurement using a ultrasonic flowmeter of the ultrasonic transducers in the form of ring, the ultrasonic wave is transmitted in the fluid along the longitudinal direction of the tube. As a result, it is described that the distance along which the ultrasonic wave is transmitted in the fluid can be prolonged by enlarging the space between the pair of ultrasonic transducers in the form of ring without considering the angle of incident and angle of refraction. It is described that therefore a flow rate of a fluid moving in a tube having a small inner diameter can be determined.
However, the measurement system employing the ultrasonic flowmeter which comprises ultrasonic transducers in the form of ring has a problem in that it is required to disassemble a part of a pre-installed tube line, if the flowmeter is arranged on the pre-installed tube lines. This is a problem also encountered in the use of a variable area flowmeter.
Further, since the ultrasonic flowmeter using ultrasonic transducers in the form of ring measures the flow rate by utilizing a ultrasonic wave which is transmitted in the fluid, in the same manner as in the measurement using the ultrasonic flowmeter of FIG. 1., if the tube is not full of the fluid or non-uniform phases such as bubbles or flowing materials are present in the fluid, the ultrasonic wave is reflected or diffused by the air phase in the tube or bubbles and the like in the fluid. Therefore, in that case, it is not possible to measure the flow rate accurately.
It is an object of the present invention to provide a method of measurement favorably employable for measuring a flow rate of a fluid moving in a tube having a small inner diameter which is utilizable without disassembling an already installed tube lines.
It is another object of the invention to provide a method of measurement favorably employable for measuring a flow rate of a fluid containing foreign phases such as babbles or floating substances.